Chapter 4

Elements, Atoms & Ions
Chapter 4
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Elements
• Over 112 known, of which 88 are found in nature
– others are man-made
• Abundance is the percentage found in nature
– oxygen most abundant element (by mass) on earth and in
the human body
– the abundance and form of an element varies in different
parts of the environment
• Each element has a unique symbol
• The symbol of an element may be one letter or two
– if two letters, the second is lower case
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Table 4.1: Distribution (Mass Percent) of the 18
Most Abundant Elements in the Earth's Crust,
Oceans, and Atmosphere
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Table 4.2: Abundance of elements in the
human body
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Dalton’s Atomic Theory
 Elements are composed of atoms
– tiny, hard, unbreakable, spheres
 All atoms of a given element are identical
– all carbon atoms have the same chemical and physical
properties
 Atoms of a given element are different from those of
any other element
– carbon atoms have different chemical and physical properties
than sulfur atoms
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Dalton’s Atomic Theory
 Atoms of one element combine with atoms of
other elements to form compounds.
– Law of Constant Composition
• all samples of a compound contain the same proportions
(by mass) of the elements
– Chemical Formulas
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Dalton’s Atomic Theory
 Atoms are indivisible in a chemical process.
– all atoms present at beginning are present at the end
– atoms are not created or destroyed, just rearranged
– atoms of one element cannot change into atoms of
another element
• cannot turn Lead into Gold by a chemical reaction
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Formulas Describe Compounds
• a compound is a distinct substance that is composed
of atoms of two or more elements
• describe the compound by describing the number and
type of each atom in the simplest unit of the
compound
– molecules or ions
• each element represented by its letter symbol
• the number of atoms of each element is written to the
right of the element as a subscript
– if there is only one atom, the 1 subscript is not written
• polyatomic groups are placed in parentheses
– if more than one
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Figure 4.2: Dalton pictured compounds as collections of
atmosphere NO, NO2, and N2O are represented
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Are Atoms Really Unbreakable?
• J.J. Thomson investigated a beam called a cathode ray
• he determined that the ray was made of tiny negatively
charged particles we call electrons
• his measurements led him to conclude that these
electrons were smaller than a hydrogen atom
• if electrons are smaller than atoms, they must be pieces
of atoms
• if atoms have pieces, they must be breakable
• Thomson also found that atoms of different elements
all produced these same electrons
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The Electron
• Tiny, negatively charged particle
• Very light compared to mass of atom
– 1/1836th the mass of a H atom
• Move very rapidly within the atom
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Rutherford’s Results
• Over 98% of the  particles went straight through
• About 2% of the  particles went through but
were deflected by large angles
• About 0.01% of the  particles bounced off the
gold foil
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Figure 4.6: (a) The results that the metal foil
experiment would have yielded if the plum pudding
model had been correct; (b) Actual results
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Rutherford’s Nuclear Model
 The atom contains a tiny dense center called the
nucleus
– the volume is about 1/10 trillionth the volume
of the atom
 The nucleus is essentially the entire mass of the
atom
 The nucleus is positively charged
– the amount of positive charge of the nucleus
balances the negative charge of the electrons
 The electrons move around in the empty space of
the atom surrounding the nucleus
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Figure 4.9: A nuclear
atom viewed in cross
section
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Structure of the Nucleus
• The nucleus was found to be composed of two kinds of
particles
• Some of these particles are called protons
– charge = +1
– mass is about the same as a hydrogen atom
• Since protons and electrons have the same amount of
charge, for the atom to be neutral there must be equal
numbers of protons and electrons
• The other particle is called a neutron
– has no charge
– has a mass slightly more than a proton
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The Modern Atom
• We know atoms are composed of three main
pieces - protons, neutrons and electrons
• The nucleus contains protons and neutrons
• The nucleus is only about 10-13 cm in diameter
• The electrons move outside the nucleus with an
average distance of about 10-8 cm
– therefore the radius of the atom is about 105 times
larger than the radius of the nucleus
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Isotopes
• All atoms of an element have the same number of protons
• The number of protons in an atom of a given element is
the same as the atomic number
– found on the Periodic Table
• Atoms of an element with different numbers of neutrons
are called isotopes
• All isotopes of an element are chemically identical
– undergo the exact same chemical reactions
• Isotopes of an element have different masses
• Isotopes are identified by their mass numbers
– mass number = protons + neutrons
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Figure 4.10: Two isotopes of sodium
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Elements
• Arranged in a pattern called the Periodic Table
• Position on the table allows us to predict properties of
the element
• Metals
– about 75% of all the elements
– lustrous, malleable, ductile, conduct heat and
electricity
• Nonmetals
– dull, brittle, insulators
• Metalloids
– also know as semi-metals
– some properties of both metals & nonmetals
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The Modern Periodic Table
• Elements with similar chemical and
physical properties are in the same column
• Columns are called Groups or Families
• Rows are called Periods
• Each period shows the pattern of properties
repeated in the next period
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Figure 4.11: The periodic table
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The Modern Periodic Table
• Main Group = Representative Elements
– “A” columns
• Transition Elements
– all metals
• Bottom rows = Inner Transition Elements =
Rare Earth Elements
– metals
– really belong in Period 6 & 7
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Important Groups
• Group 8 = Noble Gases
• He, Ne, Ar, Kr, Xe, Rn
• all colorless gases at room
temperature
• very non-reactive, practically
inert
• found in nature as a
collection of separate atoms
uncombined with other
atoms
• Noble Metals
• Ag, Au, Pt
• all solids at room
temperature
• least reactive metals
• found in nature
uncombined with
other atoms
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Important Groups - Halogens
• Group 7A = Halogens
• very reactive
nonmetals
• react with metals to
form ionic compounds
• HX all acids
• Fluorine = F2
– pale yellow gas
• Chlorine = Cl2
– pale green gas
• Bromine = Br2
– brown liquid that has lots of
brown vapor over it
– Only other liquid element at
room conditions is the metal
Hg
• Iodine = I2
– lustrous, purple solid
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Allotropes
• Many solid nonmetallic elements can exist
in different forms with different physical
properties, these are called allotropes
• the different physical properties arise from
the different arrangements of the atoms in
the solid
• Allotropes of Carbon include
– diamond
– graphite
– buckminsterfullerene
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Figure 4.18a: The three solid elemental (allotropes)
forms of carbon
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Figure 4.18b: The three
solid elemental
(allotropes) forms of
carbon
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Figure 4.18c: The three solid elemental (allotropes)
forms of carbon
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Electrical Nature of Matter
• Most common pure substances are very poor conductors
of electricity
– with the exception of metals and graphite
– Water is a very poor electrical conductor
• Some substances dissolve in water to form a solution that
conducts well - these are called electrolytes
• When dissolved in water, electrolyte compounds break up
into component ions
– ions are atoms or groups of atoms that have an electrical charge
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Figure 4.20: (a) Pure water does not conduct a
current; (b) Water containing a dissolved salt
conducts electricity
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Ions
• ions that have a positive charge are called cations
– form when an atom loses electrons
• ions that have a negative charge are called anions
– form when an atom gains electrons
• ions with opposite charges attract
– therefore cations and anions attract each other
• moving ions conduct electricity
• compound must have no total charge, therefore we
must balance the numbers of cations and anions in a
compound to get 0 total charge
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Figure 4.21a: The
arrangement of
sodium ions (Na+)
and chloride ions
(Cl-) in the ionic
compound sodium
chloride.
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Figure 4.21b: Solid sodium chloride highly
magnified.
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Atomic Structures of Ions
• Metals form cations
• For each positive charge the ion has 1 less electron than the
neutral atom
– Na = 11 e-, Na+ = 10 e– Ca = 20 e-, Ca+2 = 18 e-
• Cations are named the same as the metal
sodium
Na  Na+ + 1esodium ion
calcium
Ca  Ca+2 + 2e- calcium ion
• The charge on a cation can be determined from the Group
number on the Periodic Table for Groups IA, IIA, IIIA
– Group 1A  +1, Group 2A  +2, (Al, Ga, In)  +3
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Atomic Structures of Ions
• Nonmetals form anions
• For each negative charge the ion has 1 more electron
than the neutral atom
– F = 9 e-, F- = 10 e– P = 15 e-, P3- = 18 e-
• Anions are named by changing the ending of the name
to -ide
fluorine
F + 1e-  F- fluoride ion
oxygen
O + 2e-  O2oxide ion
• The charge on an anion can be determined from the
Group number on the Periodic Table
– Group 7A  -1, Group 6A  -2
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